1. Field of the Invention
The present invention relates in general to the field of data communications, and it more specifically relates to a packet-switched facsimile network and method of operation, with optimal utilization of network capability and point to point facsimile communication service.
2. Background Art
Digital facsimile machines using digital networks as transmission means are now increasingly being developed and implemented. Some of these facsimile machines use packet switched networks, as described in the following U.S. patents: Crager et al. U.S. Pat. No. 4,058,672; Crager et al. U.S. Pat. No. Re. 31,182; Crager et al. U.S. Pat. No. 4,058,838; Dennis U.S. Pat. No. 4,130,885; Ando U.S. Pat. No. 4,392,222; Asami et al. U.S. Pat. No. 4,841,373; and Ogawa U.S. Pat. No. 5,042,028.
Other facsimile machines use storage and forward communications techniques, as illustrated in the following U.S. patents: Crager et al. U.S. Pat. No. 4,058,672; Crager et al. U.S. Pat. No. Re. 31,182; Crager et al. U.S. Pat. No. 4,058,838; and Harvath et al. U.S. Pat. No. 5,014,300. U.S. Pat. No. 4,754,428 to Schultz et al. describes a printer protocol, which can be used in facsimile machines. All the foregoing references are incorporated herein by reference.
Other more conventional methods use point-to-point facsimile communication, where a facsimile machine transmits image data to another facsimile machine via a public telephone switching network. These methods of communications can be quite costly. In a storage and forward facsimile communications system a source or originating facsimile machine transmits image data to a first node, which, in turn, transmits the data to a second node, via dedicated lines or a packet switched network. The second node then transmits the image data to a destination or receiving facsimile machine. The use of the storage and forward communication system can be more economical than point-to-point communication. However, when the source facsimile machine finishes transmitting the image data, the destination facsimile machine will not have received the image data, which is still in the node systems.
In a packet switching communications system, a source facsimile machine, such as a Group 3 facsimile machine, can transmit image data to a destination facsimile machine, such as another Group 3 receiving facsimile machine via a packet switching network. The source facsimile machine transmits image data signals to a packet assembler/disassembler (PAD), which divides the signals into packets, and transmits these packets to a second PAD, via a packet switched network. The second PAD restores the image data signals from the packets, and transmits them to the destination facsimile machine. However, because of the difference in the transmission times of the various packets in the packet switching network, it is possible that the PAD transmits the image data signals from a packet to the destination facsimile machine, without receiving the next packet containing image data signals to be transmitted. This can interrupt the communication between the source and the destination facsimile machines, resulting in a transmission error.
As used herein, the terms "packet switching" and "packet switched" are interchangeable, and refer to a method of transferring data across a network. It divides data into segments, each of which is wrapped in an envelope to form a packet. A typical message comprises one or more packets. Each packet contains the actual user data plus information helpful to its movement across the network, such as addressing, sequencing and error control.
Packet switching is a subset of the traditional message switching (storage and forward), in which data is transmitted in blocks, stored by the first switching node it meets in the network and forwarded to the next and subsequent nodes, until it reaches the destination. No single user or large data block can tie up the circuit or node resources indefinitely.
One of the most important measure of packet switching performance is that of delay. Delay is defined for several different contexts. Cross-network delay is the amount of time a packet takes from the time it enters the network until the time it leaves the network. Such delays are typically in the hundreds of milliseconds.
The fastest rate that scanning lines can be sent to a receiving facsimile unit is determined by the minimum scan line time (MSLT), the time taken by the receiving facsimile unit to print a scan line. The standard MSLT is 20 ms, but it can range from 0 to 40 ms/line depending on the facsimile equipment design. The facsimile transmitter obtains this information from the receiver during handshake (generally in the DIS signal) and does not send faster, but often sends slower.
The actual sending time depends on the number of coded bits per line and the modem speed. The number of coded bits is determined by the amount of black and white information. MR coding gives fewer bits than MH coding. The modem speed is set during handshake. If a coded line at the transmitter is ready, the sending must be delayed by adding fill bits, a string of zeros, which are deleted at the receiver.
Therefore, there is a need for a new facsimile network including a node system, which addresses the concerns of conventional facsimile networks, and which provides adequate solutions and improvement thereto.